Automatic testing circuit



Oct. 24, 1961 w. s. OAKES, JR., ET AL 3,005,949

AUTOMATIC TESTING CIRCUIT Filed Aug. 12, 1957 M SERVO MOTOR 1/ I2 I I5 94 I7 (r SERVO 24w AMPLIFIER v PEAK LEVEL DETECTING J TRIGGER l: CIRCUIT INVENTORS. WILLIAM S. OAKES,JR. SHERMAN S- BARNHART,JR.

BY W 6/ THEIR ATTORNEY.

United States Patent 3,005,949 AUTOMATIC TESTING CHICUIT William S. Oakes, .lr., Pattersonville, and Sherman S. Barnhart, Jr., Schenectady, N.Y., assignors to General Electric Company, a corporation of New York Filed Aug. 12, 1957, Ser. No. 677,565 4 Claims. (Cl. 32420) This invention relates to a dynamic testing circuit for testing the dynamic impedance of electric circuit components simultaneously with the testing of suchcomponents for undesired defects such as short circuit and open circuit connections.

More particularly, the invention relates to a dynamic testing circuit for electron tubes for testing the current flowing in the heater filament of the tube, simultaneously with the testing of the heater filament for undesired defects such as short circuit, and open circuit connections.

The dynamic testing circuit comprising the invention is adapted to be automatically switched into electric circuit relationship with an electron tube under test seated in a socket supported on an indexing mechanism for sequentially indexing a plurality of such electron tube sockets into. and out of working relationship with the testing circuit. This testing circuit is designed to perform one of the required measurements in the production of electron tubes, and that is the measurement of the heater filament current that flows under conditions of a constant applied heater filament voltage to assure that it conforms to the specifications of the tube. Heretofore, the heater filament current of electron tubes has been established by manual testing methods using a milliammeter connected to the heater filament current with which an operator read the value of the heater filament current. Additionally, besides measuring the heater filament current, it is necessary to detect other types of electron tube heater filament defects which otherwise might cause the electron tube to fail in operation. One such heater filament defect is caused by shorted adjacent heater filament coils or folds. Most electron tubes utilize folded insulation coated wires to form the heater filaments, thereof, and usually employ from five to 17 folded turns of the wire. At each fold, the insulating material around the wire tends to break off, and expose a small section of heater wire. While the different folds or turns of the heater filament wire are designed to be staggered when the heater filament is mounted inside the cathode sleeve of an electron tube, sometime the breaks in the heater filament wire come together, and a permanent or temporary short circuit connection develops. In addition, there are other locations where either a broken or open heater filament connection might develop. This open connection may be either permanent or temporary and may occur at the point where the heater filament is welded to one of the pins of electron tube, or it may occur at any point along the heater filament wire, especially at one of the folds. In order to detect defects such as these mentioned above, it is necessary to vibrate or mechanically tap the electron tube during measurement in order to expose such latent defects. Conventional production methods utilize in addition to the milliammeter technique mentioned above, a manual tapping device, or a mechanical tapping device applied manually to uncover such latent defects which result in short or open circuit connections. i

For most electron tubes, the heater filament current with a constant heater filament supply voltage, may vary plus or minus however, it is essential that the heater filament current. magnitude fall within a specified range. Failure of the heater filament current magnitude to fall within the specified range, is cause for rejection of the electron tube under test. Also, any transient current variations occurring, such as sharp pulses in the heater filament current, are usually caused by a temporarily open or short circuited heater filament, are not acceptable under any condition, and are cause for rejection of the tube under test.

It is therefore, one object of the present invention to provide an adjustable dynamic testing circuit for testing the dynamic impedance of electric circuit components such as electron tubes simultaneously with the testing of such components for undesired defects such as short circuit and open circuit connections.

Another object of the invention is to provide a dynamic testing circuit that will meterthe current flowing through an electric circuit component, such as the current flowing through the heater filament of an electron tube, and provide a go or no-go indication of acceptance of the component under test.

A still further object of the invention is to provide a dynamic testing circuit which is responsive to either a positive going or negative going transient signal pulse to provide a no-go rejection signal in its output regardless of the steady state value of the current flowing through the component under test.

Other objects, features and many of the attendant advantages of the invention will be appreciated more readily as the same becomes better understood by the following detailed description, when considered in connection with the accompanying drawing, wherein a schematic circuit diagram of the novel dynamic testing circuit constructed in accordance with the invention, is disclosed.

The new and improved dynamic testing circuit illustrated in the single figure of the drawings is designed for use in measuring the heater filament current characteristics of an electron tube 11. The electron tube 11 has a heater filament 12 connected in series circuit relationship with an impedance 13. For example a resistor having a resistance value of one ohm has been used in a large number of instances. The series circuit thus comprised is connected to a conventional source of alternating current heater filament supply voltage having a value of one to 12 volts, and is also connected through a pair of sliding switches 14 to a variable impedance element comprising a variable resistor 15. The circuit thus formed comprises an alternating current wheatstone bridge circuit wherein the heater filament 12 forms one arm of the bridge, and the source of heater filament supply voltage, which is connected across two diagonally opposed terminals of the bridge, supplies an energizing potential for the bridge.

The remaining two diagonally opposed terminals of the bridge circuit, which are formed by the juncture of the heater filament 12 with resistor 13 and the slidable contact of variable resistor 15, are connected to the input of a servo amplifier 16. The servo amplifier 16 may comprise any conventional servo amplifier, such as any of those described on the book entitled Electronic Instruments-Greenwood, Holdham, and McRae, authors, MIT Radiation Laboratory Series, Volume 21, published by the McGraw-Hill Book Company. See chapter VIII for instance. 'It is preferred however, that the servo amplifier 16 comprise one of the'balancing amplifiers manufactured by the Brown Instrument Division of the Honeywell Regular Company such as is described in a bulletin put out by that company under the title of-- Instrument Data Sheet Number 10.20-3, entitled Servo Components or as described in Bulletin Number D15- 12 entitled Characteristics of the Brown Electronik Conitinuous' Balance System.

The servo amplifier 16 is energized from a conventional volts 60 cycle alternating current power supply source, and has its output connected to a servo motor 17. The servo motor 17 is preferably one that is designed primarily for use with the particular servo amplifier 16 (see the above mentioned descriptive literature) and intended for use in a self-balancing amplifier system. For this purpose, the motor shaft of the servo motor 17 is mechanically connected by a linkage indicated at 18 to the slidable contact 19 of variable resistor 15. When thus connected to the Wheatstoue bridge circuit, the servo amplifier 16 and servo motor 17 constitute a selfbalancing position servo mechanism connected in an alternating bridge circuit. Accordingly, when the Wheatstone bridge circuit is balanced, the angular position of the motor shaft of servo motor 17 bears a fixed relation to the value of the current flowing in the arms of the bridge circuit, and hence, to the value of the heater current flowing through the heater filament 12 of the electron tube 11 under test. The angular position of the motor shaft of servo motor 17 may therefor be used to derive an indication of the magnitude of the heater filament current flowing through the heater filament 12 of electron tube 11.

For purpose of providing a go-no-go measurement of the heater filament current two small limit switches 21 and 22 are physically disposed on opposite sides of a lever arm 23 keyed to the motor shaft of servo motor 17. These limit switches 21 and 22 are positioned so that they may be actuated by the lever arm 23, and by adjusting the position of the two limit switches with respect to the lever arm 23, the range of acceptability of the system can be adjusted. Accordingly, it can be appreciated that any disproportionate current flowing through the heater filament 12 in the bridge circuit, necessitates servo motor 17 movingthe moveable contact 19 of variable resistor a disproportionate distance so as to cause the lever arm 23 to contact with the limit switch 21 or 22, thereby providing an output indication that the long-term heater filament current of the electron tube 11 under test is not within an acceptable range as adjusted by the relative position of the limit switches 21 and 22.

The input to the servo amplifier 16, and hence the Wheatstoue bridge circuit is also connected through a conductor 24 to a peak level detecting trigger circuit 25. The peak level detecting trigger circuit is identical to a similar circuit disclosed in a co-pending United States patent application Serial No. 676,631, W. S. Oakes-inventor, filed August 6, 1957, now Patent 2,965,839 assigned to the same assignee as the present invention, and operates to develop a no-go output trigger signal that is sup plied to a grid controlled gas discharge thyratron tube having a reject relay 26 connected in its plate circuit. In the event that the magnitude of the transient pulselike signals developed in the electric component under test, for example, the signals developed in the heater filament of an electron tube under test 11, by reason of the tapping of the same and the occurrence of a latent short or open circuit connection, exceed some preselected value, the peak level detecting trigger circuit 25 actuates reject relay 26 causing it to reject the electric component under test.

In placing the novel dynamic testing circuit in operation, the electrical components to be tested, in the present example electron tubes 11 are indexed into position, and the necessary electrical contacts made thereto through the sliding switches 14. Simultaneously, the electron tube 11 under test is preferably physically disposed in operating relationship with a suitable tube tapping mechanism.

Immediately after all of the required circuit connections are completed, the heater filament 12 of the electron tube 11 under test is connected in electric circuit relationship in the above described Wheatstoue bridge circuit, and the self-balancing servo mechanism immediately drives the moveable contact 19 of variable resistor 15 to a point required to achieve balance of the bridge circuit. In the event that the tube under test is normal, and the magnitude of heater filament current thereof lies Within an acceptable range, the null position of the Wheatstoue bridge will be maintained for the duration of the test, and the tube will be accepted. If the heater current of the tube is too high or too low, either one of the limit switches 21 or 22 will be actuated, thereby en ergizing a reject circuit (not shown) for providing an output indication that the tube under test is not acceptable, or for physically ejecting the tube from its supporting socket. Simultaneously with the above described heater filament current measuring operation, the tube under test 11 is being mechanically tapped, and the pres ence of any latent defects such as open circuit or short circuited adjacent turns in the heater filament, will result in the production of a sharp transient signal pulse supplied to servo amplifier 16. Such sharp transient signal pulses because of their transient nature may not be sufiicient to operate the high inertia, slow response servo system comprised by servo amplifier 16 and servo motor 17, and hence will not serve to readjust the balance position of the Wheatstoue bridge circuit. Also, in the case of permanent or semi-permanent shorted or open heater filament turns, a sharp or pulse-like increase in the steady state value of the heater filament current may occur, which is insuificient to cause actuation of the limit switches 21 and 22. For this purpose the sharp transient signal pulses are also supplied by the conductor 24 to the peak level detecting trigger circuit 25, and if they are sufficient amplitude, the reject relay 26 will be actuated, and the tube 11 under test will be rejected.

mm the foregoing description, it can be appreciated that the invention provides a new improved dynamic testing circuit for testing the dynamic impedance of electric circuit components such as electron tubes, simultaneously with the testing of such components for undesired defects such as short circuit and open circuit connections. This new and improved testing circuit serves to meter the current flowing through the electric circuit component, such as the current flowing through the heater filament of an electron tube under test, and to provide a go or no-go indication of acceptance of the component under test. It can be further appreciated that the invention provides a dynamic testing circuit which is responsive to undesired transient signal pulses occurring in the component under test to provide a no-go rejection signal in its output regardless of the steady state value of the current flowing through the component.

Obviously, other modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiment of the invention described herein which is Within the full intended scope of the invention as defined by the appended claims.

What We claim as new and desire to secure from the Letters Patent of the United States is:

1. A dynamic testing circuit for testing the dynamic impedance of electric circuit components simultaneously with the testing of such components for undesired defects such as short circuit and open circuit connections comprising an impedance bridge circuit adapted to have the electric circuit component under test connected in one arm thereof and having a variable impedance in another arm thereof, means for connecting a source of electric potential across the input of said bridge circuit, a servo amplifier fed by the output of said impedance bridge circuit, a servo motor controlled and drivingly connected to vary said variable impedance for maintaining the bridge circuit in a balanced condition during testing, a limit switch actuator driven by said servo motor, a pair of limit switches in the path of said limit switch actuator for actuation thereby upon variation of said variable impedance to predetermined limit values a continuously operating fast response peak level detecting transient responsive trigger circuit continuously operatively coupled to the electric circuit component under test, and a reject relay connected in the output of said trigger circuit for providing an output indication of the occurrence of transient electric signals developed in the component under test which exceed a preselected magnitude.

2. The combination set forth in claim 1 wherein one arm of said impedance bridge circuit is comprised by the electric circuit component under test connected in series electrical circuit relationship with a second impedance, and the remaining arm of said bridge circuit is comprised by a variable impedance With the input to the servo system being connected to the juncture of the component under test and the second impedance, and to the moveable contact of the variable impedance.

3. In an electron tube test circuit adapted to be automatically switched into electric circuit relationship with an electron tube under test seated in a socket supported on indexing mechanism for sequentially indexing a plurality of such electron tube sockets into and out of working relationship with the testing circuit and to position the electron tube under test in working relationship with respect to an automatic tube tapping mechanism for mechanically tapping the tube to uncover latent defects in the construction thereof, the combination comprising a resistance bridge circuit adapted to have the heater filament of the electron tube under test connected in one arm thereof, means for connecting a source of electric potential across said bridge circuit, a high inertia selfbalancing servo system connected across said resistance bridge circuit for maintaining the bridge circuit in a balanced condition during testing, indicator means connected to said servo system for indicating the servo system to be operating out of a preselected range of balancing conditions in order to affect balancing of the resistance bridge circuit, and a fast response peak level detecting trigger circuit operatively coupled to the electric circuit component under test and having a reject relay connected in the output thereof for providing an output indication of the occurrence of transient electric signals developed in the component under test which exceed a preselected magnitude.

4. The combination set forth in claim 3 wherein one arm of said impedance bridge circuit is comprised by the heater filament of the electron tube under test connected in series electrical circuit relationship with a second resistor, and the remaining arm of said bridge circuit is comprised by a variable resistor with the input to the servo system beng connected to the juncture of the heater filament of the tube under test and the second resistor, and to the moveable contact of the variable resistor.

References Cited in the file of this patent UNITED STATES PATENTS 

